1. Field of the Invention
The present invention relates generally to motor drive circuitry, such as may be used in magnetic tape storage devices for computer applications, and, more specifically, to a method and apparatus for making low inertia motors useful in tape drives.
2. Description of the Related Art
From their inception, digital computers have required some form of data storage as an adjunct to their relatively sparse main memory facilities.
Magnetic tape devices are a fast, efficient, economical means for storing computer data such as for backing-up hard disk software application programs and user data created using the programs, or for off-line file management tasks routinely implemented between a disk and tape as the data is processed. These devices, using tape cartridges with 1/4 inch, 8 mm, and 4 mm tape, are available for storing data in capacity ranges that match the requirements of small to medium computer systems. For example, commercially available devices can store up to 525 megabytes of digital data on quarter-inch tape in cartridges which load into 5 1/4 inch form factor, half-height drives that can be plugged into the same slot that a floppy disk drive can be installed.
Streaming magnetic tape drives, also called streamers, are constant speed transports for storing information from hard drives to provide backup. Such drives generally record bidirectionally, laying down as many parallel tracks as tape head technology will permit; current technology is about twenty-six tracks on quarter-inch tape. A description of computer tape drive technology can be found, for example, in STREAMING, Copyright 1982, Archive Corporation, Library of Congress Catalog No. 82-072125.
In order to reduce power consumption during a drive's start-up sequences, it is preferable to use low inertia motors. Moreover, such motors are smaller and more economical. However, a motor with low inertia, in comparison to the load it must accelerate, would cause acceleration to vary proportionally with the load. Consequently, the time to reach operating speed would also vary proportionately with the load.
In accordance with specifications, generally established by groups such as the American National Standards Institute (ANSI) and the Quarter-Inch Committee (QIC), the load on the motor caused by a QIC tape cartridge can vary from approximately one ounce to approximately 5.5 ounces. Therefore, drag will vary from tape cartridge to tape cartridge. Additionally, drag can change with the age of the cartridge depending on its frequency of use and an operational temperature conditions. Moreover, the drag on the motor will vary directly with the tape speed.
Since the load of the cartridge varies at a ratio of approximately 5.5:1, the start-up time of the tape drive can vary five-hundred and fifty percent if a low inertia motor is used. This makes it difficult to meet the worst-case "up to speed time" of 300 milliseconds as established in the QIC industry standard equipment specification.
Moreover in any case, the maximum acceleration a cartridge is specified to withstand is 1,500 inches/second/second. Thus, if a drive motor current limit is set such that a high drag cartridge comes up to speed within the worst-case time limit, a low drag tape inserted the same drive be destroyed since five and one-half times the amount of necessary energy is supplied on start-up. On the other hand, if the current limit is set such that a low drag cartridge will start-up well within the QIC specification, a high drag cartridge will only start-up well beyond the specification time limit.
Clearly, the benefits of low inertia motors--equipment miniaturization, reduction of power consumption, economy--are offset by the problem of meeting the acceleration specifications.